Hi. This post desvribes some long experiments and learning. If you want to see the results than scip the next 20 posts.

hello together
I finished my first "real" oscillator today, it´s this one:
http://www.musicfromouterspace.com/analogsynth/August2003VCO.html
Again, my best regards to ray!
Well, first nothing was working, till i noticed i had to turn the fet. The bf245 has opposite pinout to the mpf102.
After that everything worked, so that was the fastest troubleshooting i ever had.
Just those waveshapers, they drive me crazy!
first of all, i tried different integrator cabs. With small values i get some strange slow fm on the output. With bigger cabs, like 20-330n (instead of 330p) it gets stable. Right now i use 20n.
Next thing is the spike i get at the bottom of the triangle. seems to be better with small integrator cabs, but as i said, it shakes a bit.
So i tried different cabs across R27, but it won´t really do. Either the spike remains, or it disappears, while turning the tri into a saw with little amplitude (with cabs around 200n).
Because of that it´s also impossible to get a good sine.
I try to get the sine by trimming the tri as good as i can. than i set r15 (i use a trimmer) so that the sine out is completely squared. Adjust it to be symmetric, then raise r15 till it looks like a sine. Only thing is it doesn´t sound like one. I still got a lot of overtones that i can´t get rid of, and i think it´s because of that spike.
I could imagine that some of you have already gone through this, so if you can help i´m glad and happy
Otherwise i won´t need a barber anymore in a couple of days...Last edited by zipzap on Tue Jul 25, 2006 7:01 am; edited 6 times in total

Ok, ray talks about it himself, it´s hard to get a good sine and impossible to get rid of the gliches.
Still all that integrator cab buissenes i mentioned is strange. Maybe it´s because i use styroflex or mks foil?
Also, do you know a better sineshaper?

Now i´m getting exited. I noticed that with the trimpots i can get the sine out to sound like any of the waves. Let´s start by hooking up the tri out. Saw offset can be used to morpf from saw(rising ramp) to triangle. At the Sine Out tri-offset controlls the symmetrie, R15 can be tweaked to go from Tri to sine to square, or Ramp to sinish ramp to pulse, or anything inbetween.
Way cool, dialing in as much odd or even harmonics as i want, instead of fixed waveforms. AC-Coupling can get rid of the offset.
This would be a really cool lfo as well, hope i can elliminate that glich so it won´t bleed through when modding something.

Ok, another thing that should be done to keep pwm working is to get another buffer-amp for the raw saw (can i just go from the point marked RW with say another 100k to another opamp or will that change everything?). Than that saw can be used for PWM. Guess it should work if i take that additional buffer, maybe add some offset and send it directly to R26 (which has to be removed from the tri-waveshaper). So then that comparator will always see the same saw, even if i change all the other offsets.

The triangle spike is common with a saw core vco, I think I saw one on the Blacket VCO scope trace that Bigtex posted. A triangle core vco can produce near perfect triangle (and therefore better sine). Also, it does not require HF trim because it has no constant reset time to compensate.
I suspect that a tri > saw wave shaper could produce a better sawtooth wave as well.
The sawtooth design is simpler though and I don't think the glitch causes many problems in real use, once it gets passed through a filter etc.

Have you tried using styrene caps? Are Styroflex the same as styrene? I heard somewhere that for some VCO caps, only styrene are good enough. Just an idea. _________________What makes a space ours, is what we put there, and what we do there.

Don´t know, styroflex is a brand name, it´s a Polystyrol foil type, they are said to be extremely temperature stable and high quality. Problem is that in german some of these types have different names. Doesn´t make things easier. I posted in a german electronics forum, they told me that with integrators all types should work, with the ones i tried (MKS, MKP, Styroflex) giving good temp and frequency stability. They also told me that with smaller values unwanted effects from the outside, like psu noise/oscillation become a bigger concern. I use Ray´s PCB layout, but with a lot of wires hanging out to check different components. That might be a problem by itself. Right now i use a 0,047 MKP and everything seems stable. Range is good also.
About the spike, well it brings in some additional overtones, so pure sine is impossible. A 10nf cab across R27 cured the problem very efficiently, but then when i tune the whole thing to ramp, some of the overtones are missing. Without the cab the triangle out ramp (with saw offset to start from ground) sounds exactly like the saw out.
So i have different possibilities. Use a switch to cab or not to cab.
Or have a pure saw out plus a more dampened, darker saw-sine-tri out, or both. Getting more and more complex, but i think in this case it´s worth the extra panel space.
I guess i´ll also keep the different outs (Saw, Pulse, multi), to mix them or send them do different filters and stuff.
The reason why i really want to get rid of the spike is for LFO use. When i saw on the scope what all those trimmers can do to the waveform i know that this is the perfect lfo as well. Having a range switch for lfo mode, that could be a Double switch, so that the cab is only connected in lfo mode.

Another thing is that i´m now thinking of crowding the panel even more by adding some vc-ins for the waveshapers. Guess thats where it really gets fun.
EG, i can see how the saw-offset is added in the bottom left of the schematic. If those tri and saw trimmers were going to the inverting ins of their opamps i would know what to do. But they are going to the noninverting ins. How can i add another cv-in at those points? Do i have to add a complete two Opamp mixer for Pot and cv-in and connect it, in the case of the saw offset, to IC3D, Pin12? Or is there a simpler way?

I have heard of the advantages of tri-core vcos, although i doubt that it will make a better saw! For some reasons they are not very common, at least i never found a shematic. Are they really that complicated?
I think i´ve seen them in vc-lfo schematics, like the formant one.
The best sine i ever had on my scope and speaker was from the xr2206 and from my modded state variable sine/cosine oscillator.
With this vco, the best sine i could get was very close to the triangle. That´s why i decidet to use only the "sine"-out and have a pot to more or less squeece it from tri to "sine" to square.
But what you say about the filter is really true! So if i want a pure sine sequence with no filter i can still use the other osc!

Although this Forum is for discussion, i have to answer myself once more.
I ended up using a 4n styroflex, the spike is almost gone and the shaking as well. must have been the cab hanging to fahr away with alligator clips for experimenting.
Ok, here are three little samples:
1 Tri to sine to square
2 Saw to square
3 somewhat tri to kind of saw.
They all looked better on my scope, that must be because of tube preamp and soundcard, but actually it doesn´t matter what it looks like.
But now the big point is this: The first two examples were made with fixed offset trimmers and by adjusting R15 between maybe 400k to 0 ohm, thereby setting the overdrive of the ota. This results in changing amplitude. I limited them in the pc. I would really like to make this voltage controlled, but i don´t know how. Could be fet, ota or vactrol i guess.
Do you think it is possible to compensate the voltage drop? Maybe two vactrols being controlled at the same time. When the light gets brighter one makes more (louder) square, the other is at the output going to ground, turning the volume down at the same time. I know there are filters with q-compensation using a vca, so i guess it is possible. The question then again is how. Do i have to just start trying different values and methods, or can you suggest a more systematic way to get to a good result?
Maybe even the Ota that´s already on the board could be used. There is one side unused, so that could take the part of R15. Then the Amplitude Pin1 of the OTA doing the sineshaping is tied to +12v via 390k. Maybe it could be the vca by itself. Don´t know, propably not.
Ok guys, many questions i know, but maybe someone has an idea!

Hi Zipzap,
I must admit that I'm the kind of person who is way too easily pleased that something works at all!
I'd probably say, well there's sound coming out, and it stays in tune, great!
But admittedly I get picky about other things, like front panel aesthetics, and neat wiring.
I've virtually never tried controlling the things you're trying to control, so I can't really say much about that, except that I'd try vactrols and a lot of experimentation. Check with Scott, he's the vactrol master. If I wanted to stop Indigo from throwing her dinner all over the kitchen floor, I'm sure Scott would work out a way of doing it with vactrols!! _________________What makes a space ours, is what we put there, and what we do there.

I must admit that I'm the kind of person who is way too easily pleased that something works at all!

Me to! (normaly) But this time it´s different! Untill now i was mostly using vactrol square oscillators and was always exited about the fact that i had sound going on. Then i finally decidet that it was time to build a real vco. I hand copied Rays layout with pen and copper, drilled, got the parts (free ssm2210) and everything worked right from the start (after turning the fet the right way). That has never happened and now i think that i can do everything (except neat wiring, on the inside my stuff looks like a pile of garbage)
Greetings to Indigo! (nice name)

If I wanted to stop Indigo from throwing her dinner all over the kitchen floor, I'm sure Scott would work out a way of doing it with vactrols!!

I'm drawing a blank here. I had to nix the Vactrol guided spoon (too many wires, uncomfortable body harness), the cage of LED's (requires Indigo to eat in near to total darkness) and the rug of interwoven VTL5C3/2's (just too expensive). I an at a bit of of a disadvantage - I'm sitting in the Corpus Christi International airport with a fading computer battery, so there's that bit of pressure.......

I downloaded the samples, but I'm not in a position to listen to them yet, can't wait to hear them, ZipZap!

hi guys
I´ve been buissy like a bee this morning, too hot to go out. I´m happy to tell you that this is getting somewhere! I decided to start with adding voltage control to sweep from tri to saw using the saw offset.
When you look at the shematic there is the saw-offset-opamp in the lower left. I disconnected pin 12 of that IC3-D and hooked it to a pot, dividing between 0 and 12v. With that voltage the tri-out can be sweeped between rising ramp, tri and falling ramp. The problem is that by the full wave recifiction the tri has only half the amplitude. Further down the path there is R25 setting the gain of the tri wave. Normally the ratio of R28 (100k) and R25 (200k) doubles the amplitude of the tri-shaper, so that the tri out is as loud as the saw out.
In other words, the voltage at pin 12 and the value of R25 have to be changed at the same time.
First i build a vactrol circuit like this: Vactrol from +12 to pin 12, Trimpot from Pin12 to ground. Another vactrol goes across R25. The leds are in series. So by making the leds brighter the volume goes down.
Ok, like that sweeping from falling ramp to rising ramp doesn´t make sense, because in the first half the amplitude compensation would work in the opposite way. So i decided, since falling and rising ramp sound the same, to have a vc-sweep from tri to saw.
When the 3 Low current Leds get about 0,3ma i get a triangle. By rising the current to about 0,515ma turns it into a saw.
What i really didn´t expect was that the amplitude compensation worked just as it should from the start, with no trimming! Still got some very little amplitude change, but it´s hardly noticable! If i want to optimize that, i guess i would have to adjust R25 R28 so that the two extremes of the vactrol resistance varies the amplification exactly between 1 and 2.
But as i said, it´s already almost perfect!
I post the led driver i currently use. The things that are different are 3lc leds + a 10k resistor in series. Also i have to adjust the input resistor so that a sweep from 0 to 10v results in those 300-515ua.
So fahr so good.
Now i need your help: How can i ensure that the led current doesn´t go below 300ua? The panelmounted pot can easily be trimmed for the range, but what about cv from the outside world? If that is negative and the manual pot is set to a low voltage the sum of those two voltages could easily turn the led too dark. If it does go below 300ua it turns the tri towards falling, ramp, at the same time rising the amplification, resulting, as i said, in the opposite of the compensation i want and into clipping.

A lot of writing here, i know. When it´s all finished i will post a shematic for the whole mod that will say more than 1000 words!
Till then thanks for reading, thinking, contributing!Last edited by zipzap on Fri Jul 14, 2006 6:09 am; edited 1 time in total

This might be a really stupid idea, if it is I'll hopefully learn why. What if you put the LEDs supply through a rectifier of some kind. With a large(ish) cap to keep it warm when you're between -.7&+.7V No doubt it wouldn't be that simple, but maybe the idea behind it is a clue?
The only other idea I've got is to reference the LED from your -ve rail, so moving through 0V won't mean diddly squat.
Actually, on that second idea, can't you use an op amp to apply an offset which would convert -10V->+10V into +2V->+5V or something along those lines?_________________What makes a space ours, is what we put there, and what we do there.

I´m sorry, but i don´t understand. I´m afraid none of your ideas.
I need a led driver that makes the led current stay between 0,3 and 0,5 ma (values may change, since maybe i´ll do some more trimming). I guess this is kind of an general problem with bipolar and unipolar controll voltage. Like with pwm- circuits. At some manual settings a cv may cause the pulse to go off.
Here it´s somewhat like this thing expecting a cv going from ground to say 10v (depending on how it´s trimmed). Ground means tri, 10v means saw.
if i set the manual knob to tri or too close to tri and apply too much of a bipolar cv my led´s will get too dark. Then i have a problem because i would get a boosted ramp. Instead i want that in this case nothing happens once the current has dropped to where it gives me the triangle.
If that´s possible would be cool, otherwise i have to watch those settings during operation (wich is ok, too)

I like how that sounds, but i´m afraid the vactrol solution is useless. This is just much to unprecise, once set to tri the waveform starts to drift within some minutes. Now gonna try the obvious way: Offsetvoltage applyed by cv-mixer, amplitude compensation with vca.

Ok, if someone likes to include this in their vcos, here is part 1 of the mod.
vc tri to saw is realized by shifting the offset voltage at pin 12 with that cv-mixer. The offset-trimpot is set for triangle output with no cv connected. The cv-summing resistors R1,2 set the range of the cv, so that at max cv-input you get a saw.
Now it gets a little tricky again. I´m, not good with otas, so i stayed with vactrols for the amplitude compensation. I used the Leddriver posted above and started trimming R3 and R4 until the vactrols resistance was changeing between 100k and 200k, when changing from tri to saw.
Then replaced R25 with that vactrol. Not the most accurate way, since vactrols are not very linear, but somehow it works fine. Any suggestions?
Guess i´ll go on with the vc tri-sine-square than.

Still one problem remains: The voltage at pin 12 must stay between about 1,5 and 3,1v. There must be some way to limit the output voltage of that cv-mixer to that range. I just don´t know how. With zener-diodes?

Yes, that spike is there, but it´s hardly audiable. I realized that it depends largly on the size of the integretor cab, so after going back to smaaler cab volumes it was almost gone.
What worries me more right now is that i can´t limit that offset cv. I tried what´s in the schematic posted, trimming those two trimmers to the min and max voltage levels. I thought now the diodes would take care of everything. Well, it works fine for the voltage going to high. I used a red led and it startet to go bright when the voltage went to high (below -3v, since it´s inverted by the first opamp). The other trimpot is set to -1,5v, so i´m just able to get a triangle wave. but with negative cv applied it still goes above those 1,5v. Don´t know why.

and here is the updated shematic. Buld it today and it works. But trimming that vactrol to keep the amplitude constant is a pain in the ass! I ended up using a 47k series resistor. After all it´s not that critical. If the amplitude changes a bit it´s not so bad. The saw sounds louder anyway and like someone posted elsewhere our ears are terrible for measuring amplitude. I managed to keep it quite constant on the scope.

Hi guys!
Today i went on to the vc tri to sine to square. A vactrol alone doesn´t work, cause it doesn´t get the range needed to drive the lm13700 from no distortion to squaring.
So i found this circuit (the top one) and it works. The opamp-feedback is very low compared to the vactrol, the output goes to the ota directly.
The points sc, sine and ic2a are on the ray wilson august2003 vco schematic.
Now i need your halp: The amplitude changes in a very nonlinear way: big change between tri and sine, little change between sine and square.
I think it would be best to use a dynamic limiter at the output. I think it could look somewhere like the second circuit, but i can´t find the right values. It just won´t work. I´m really not into otas, never used them besides from finished designs, so i don´t have a clue what to do right now. If you have any idea that would be great! this is the last step to the finished Mod!

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